Coating composition

ABSTRACT

A coating composition comprising 
     (A) a film-forming polymer having functional group(s) capable of reacting with the crosslinking agent (D) hereinafter referred to and cationic or anionic functional group(s), 
     (B) a volatile organic liquid diluent in which the polymer (A) is dissolved, 
     (C) crosslinked polymer microparticles bearing anionic or cationic functional group(s) whose electric charge is opposite to that of the film-forming polymer, which are insoluble in the combination of the film-forming polymer (A) and the diluent (B) and are maintained in a stabilized state of dispersion therein, and 
     (D) a crosslinking agent dissolved in the diluent (B).

FIELD OF THE INVENTION

The present invention relates to a coating composition and morespecifically, it concerns a coating composition containing crosslinkedpolymer microparticles, which is particularly useful as decorativecoating for automobile bodies and other articles.

BACKGROUND OF THE INVENTION

Recently, a high solid coating composition comprising crosslinkedpolymer microparticles dispersed in a conventional coating compositioncontaining film-forming resinous vehicle, has been watched with keeninterest in various coating areas and especially in an automobileindustry, because of the eminent workability and capability of obtaininga coating with excellent film properties. These particles are composedof polymer crosslinked to the extent that it is insoluble in an organicsolvent in which the particles are dispersed, finely pulverized tomicron size, and stably dispersed in the coating composition. Dispersionof said microparticles in a solvent or a carrying vehicle is oftencalled, for convenience, as "microgels". However, when microgelparticles and coating resinous vehicle are lacking in mutual affinity,there are such problems that thus obtained coating composition willgenerate precipitations through agglomeration of said microgel, duringstorage and there give troubles in gloss and smoothness of the resultedcoating likewise through agglomeration of microgel particles in bothcoating and drying steps. Employment of an emulsifier or dispersingagent will obviate said agglomeration, but it will cause additionalproblems of poor qualities of the formed coating such as water proofnessand the like.

Attempts have therefore been made to settle the abovementioned problems,to get a far stabilized dispersion of polymer microparticles in resinousvehicle with the help of various specific type emulsifiers; however, nosatisfactory solutions have been found yet. Under the circumstances, itwould be an immeasurable advancement of the technical level concerned ifone could provide a highly stabilized dispersion of microgel particlesin a resinous vehicle without relying on a specific type of emulsifierand provide a high solid coating composition based on said dispersion,which is excellent in storage stability and capable of resulting in acoating with excellent gloss and improved appearance.

SUMMARY OF THE INVENTION

The principal object of the present invention is, therefore, to providea high solid coating composition containing crosslinked polymermicroparticles in a stabilized state of dispersion therein, which isexcellent in storage stability, capable of resulting in a coating withexcellent gloss and appearance, and which is specifically useful as adecorative coating for automobile bodies and other articles. The otherobjects of the invention will be apparent from the description of thespecification and accompanying claims. According to the presentinvention, the abovesaid objects can be attained with a coatingcomposition comprising

(A) a film-forming polymer having functional group(s) capable ofreacting with the crosslinking agent (D) hereinafter referred to andhaving cationic or anionic functional group(s),

(B) a volatile organic liquid diluent in which the polymer (A) isdissolved,

(C) crosslinked polymer microparticles bearing anionic or cationicfunctional group(s) whose electric charge is opposite to that of thefilm-forming polymer, which are insoluble in the combination of thefilm-forming polymer (A) and the diluent (B) and are maintained in astabilized state of dispersion therein,

and (D) a crosslinking agent dissolved in the diluent (B).

The inventors have found that when anionic or cationic functional groupsare introduced in a film-forming polymer to be used as a resinousvehicle in a coating composition and cationic or anionic functionalgroups whose electric charge is opposite to that of the functional groupof said film-forming polymer are introduced in crosslinked polymermicroparticles, an extremely stable dispersion can be obtained with thecombination of said film-forming polymer and said crosslinked polymermicroparticles, since an adsorption layer of said film-forming polymeris formed on the whole surface of the respective microparticles throughan electrostatic interaction of these functional groups. The inventorshave also found that the thus obtained coating composition is excellentin storage stability and is excellent in gloss and appearance. On thebasis of these findings, the invention has been made.

PREFERRED EMBODIMENTS OF THE INVENTION

The film-forming polymer constituent (A) of the composition of thisinvention may be any polymer known to be useful in coating compositions,provided at it has a functional group capable of reacting with thecrosslinking agent (D) hereinafter referred to as, for example hydroxylgroup, and it has a cationic functional group such as, for example,amino group, mono-substituted amino group, di-substituted amino group orammonium group or an anionic functional group such as, for example,carboxyl group, sulfonic acid group or phosphoric acid group.

Examples of such polymers are acrylic, alkyd and polyester resinsbearing such two kinds of functional groups. Usually, they have an acidvalue of 0.5 to 60, hydroxyl number of 20 to 200 and number averagemolecular weight of about 500 to 10,000.

To attain or promote hardening of said polymer (A), a crosslinking agent(D) is compounded together. This may be any of the known crosslinkingagents customarily used in the related field, providing that it issoluble in an organic liquid diluent (B) and is capable of reacting withthe abovementioned functional group of the film-forming polymerconstituent (A). Appropriate members are, for example, diisocyanates,diepoxides and aminoplast resins. Particularly preferable ones aremelamineformaldehyde condensation products, substantial portions ofwhose methylol groups are etherized with butanol or methanol. Theabovesaid film-forming polymer (A) is carried in a volatile organicliquid diluent (B) in the form of dispersion or solution, or may bepartly in dispersion and partly in solution, though the crosslinkingagent (D) is necessarily dissolved therein. Examples of such volatileorganic liquid diluents are aromatic hydrocarbons such as toluene,xylene and petroleum fractions of various boiling point ranges having asignificant aromatic content, esters as butyl acetate, ethylene glycoldiacetate and 2-ethoxyethyl acetate, ketones such as acetone and methylisobutyl ketone, alcohols such as butyl alcohol, aliphatic hydrocarbonsand mixtures thereof.

Besides the abovementioned film-forming polymer constituent (a),crosslinking agent (D) and volatile organic liquid diluent (B), thecoating composition of this invention, as the most characteristicfeature thereof, contains novel polymer microparticles (C) hereinunderdefined. The polymer microparticles (C) present in the composition ofthe invention are crosslinked polymer microparticles bearing anionic orcationic functional groups whose electric charge is opposite to that ofthe functional groups of said film-forming polymer, which are insolublein the combination of the film-forming polymer (A) and the diluent (B)and are maintained in a stabilized state of dispersion therein. From thestandpoint of preparation easiness, particularly preferable members arecrosslinked acrylic or vinyl resin microparticles. Such microparticlesmay be easily and advantageously prepared by the copolymerization of atleast 3 α,β-ethylenically unsaturated monomers, the first member beingα,β-ethylenically unsaturated compound having an anionic or cationicfunctional group, the second member being crosslinking monomer(s) whichis either a polyfunctional monomer having two or more ethylenicalunsaturations in its molecule or combination of two differentethylenically unsaturated compounds each having mutually reactivefunctional group, and the third member being a α,β-ethylenicallyunsaturated monomer other than the abovesaid first and second monomers,in an aqueous or organic medium in the presence of a dispersing agent.As the α,β-ethylenically unsaturated compound having an anionic orcationic functional group, mention is made of

(1) carboxyl bearing monomers such as, for example, acrylic acid,methacrylic acid, crotonic acid, itaconic acid, maleic acid, fumaricacid and the like,

(2) sulfonic acid bearing monomers such as, for example,2-acrylamide-2-methylpropane sulfonic acid, styrene sulfonic acid andthe like,

(3) nitrogen-containing alkyl acrylates or methacrylates such as, forexample, dimethylaminoethyl acrylate, dimethylaminoethyl methacrylate,dimethylaminopropyl methacrylate and the like. Besides the above,phosphoric acid bearing monomers and ammonium group containing monomersmay likewise be used. Crosslinking monomers may be any of the knownmonomers of functionality greater than two. Preferable members are theso-called polyfunctional monomers having two or more ethylenicalunsaturations in its molecule such as polymerizable unsaturatedmonocarboxylic esters of polyhydric alcohol, polymerizable unsaturatedalcoholic esters of polycarboxylic acid and aromatic compoundssubstituted with two or more vinyl groups. Examples of such members areethyleneglycol diacrylate, ethyleneglycol dimethacrylate,triethyleneglycol dimethacrylate, tetraethyleneglycol dimethacrylate,1,3-butyleneglycol dimethacrylate, trimethylolpropane triacrylate,trimethylolpropane trimethacrylate, 1,4-butanediol diacrylate,neopentylglycol diacrylate, 1,6-hexanediol diacrylate, pentaerythritoldiacrylate, pentaerythritol triacrylate, pentaerythritol tetraacrylate,pentaerythritol dimethacrylate, pentaerythritol trimethacrylate,pentaerythritol tetramethacrylate, glycerol diacrylate, glyceroldimethacrylate, glycerol allyloxy dimethacrylate,1,1,1-tris-hydroxymethylethane triacrylate,1,1,1-tris-hydroxymethylethane dimethacrylate,1,1,1-tris-hydroxymethylethane trimethacrylate,1,1,1-tris-hydroxymethylpropane diacrylate,1,1,1-tris-hydroxymethylpropane triacrylate,1,1,1-tris-hydroxymethylpropane dimethacrylate,1,1,1-tris-hydroxymethylpropane trimethacrylate, triallyl cyanurate,triallyl isocyanurate, triallyl trimellitate, diallyl terephthalate,diallyl phthalate, and divinyl benzene.

Crosslinking monomers may also be the combination of two ethylenicallyunsaturated monomers each bearing mutually reactive functional groupsuch as, for example, the combination of epoxy bearing ethylenicallyunsaturated monomers, such as glycidyl acrylate and glycidylmethacrylate, and carboxyl bearing ethylenically unsaturated monomers,such as acrylic acid, methacrylic acid and crotonic acid. Other examplesof combinations of mutually reactive functional groups are amine andcarbonyl, epoxide and acid anhydride, amine and acid chloride,alkyleneimine and carbonyl, organoalkoxysilane and carboxyl, hydroxyland isocyanate, and the like.

In this invention, other α,β-ethylenically unsaturated monomers than theabovesaid first and second groups may be copolymerizable therewith.Examples of such monomers are as follows:

(1) hydroxyl bearing monomers as, for example, 2-hydroxyethyl acrylate,hydroxypropyl acrylate, 2-hydroxyethyl methacrylate, hydroxypropylmethacrylate, hydroxybutyl acrylate, hydroxybutyl methacrylate, allylalcohol and methallyl alcohol,

(2) polymerizable amides as, for example, acrylic amide and methacrylicamide,

(3) polymerizable nitrites as, for example, acrylonitrile andmethacrylonitrile,

(4) alkyl acrylates or methacrylates as, for example, methyl acrylate,methyl methacrylate, ethyl acrylate, n-butyl acrylate, n-butylmethacrylate and 2-ethylhexyl acrylate,

(5) polymerizable aromatic compounds as, for example, styrene, α-methylstyrene, vinyl toluene and t-butyl styrene,

(6) α-olefins as, for example, ethylene and propylene,

(7) vinyl compounds as, for example, vinyl acetate and vinyl propionate,

(8) diene compounds as, for example, butadiene, isoprene and the like.

These monomers are used alone or in combination. These three groups ofα,β-ethylenically unsaturated monomers are reacted in an aqueous ororganic liquid medium in the presence of a dispersing agent oremulsifier in a conventional way, to obtain microparticles ofcrosslinked copolymer bearing anionic or cationic functional groups,whose electric charge is opposite to that of the functional groups ofthe film-forming polymer.

In this invention, the polymer microparticles (C) and the film-formingpolymer (A) are electrostatically combined together by the functionalgroups carried on the respective polymers, and hence the whole surfacesof the polymer microparticles (C) are covered with the film-formingpolymer (A) and such microparticles can be stably dispersed in thecombination of said film-forming polymer (A) and the diluent thereof.Therefore, there is indeed the necessity of using a certain amount ofemulsifier or dispersion colloid for the preparation of said crosslinkedpolymer microparticles through an emulsion polymerization or NAD method,but no need of using a particular type of dispersing agent or emulsifierfor the dispersion of thus obtained crosslinked polymer microparticlesin a dispersing medium. Thus, in the present invention, a relativelysmaller quantity of emulsifier is used only for the purpose ofpreparation of said microparticles and therefore there is the leastadverse effects on the coating.

As the emulsifer, any of the members customarily used for an emulsionpolymerization or NAD polymerization of α,β-ethylenically unsaturatedcompounds may be successfully used in the least effective amounts.However, from the standpoint of obtaining a better quality coating, itis more desirable that the emulsifer is of the nature of high polymergiving no or the least harmful effects on the formed coating. Thus, inthe most preferable embodiment of the invention, the polymerization ofα,β-ethylenically unsaturated monomers is carried out in the presence ofa resin having an amphoionic radical of the formula: ##STR1## in which Ris a member selected from optionally substituted alkylene having 1 to 6carbon atoms and phenylene groups, and Y is --COOH or --SO₃ H, as anemulsifier as stated in Japanese Patent Application Kokai No. 129066/83.

Examples of such resins are alkyd, polyester, modified epoxy, acrylic,melamine and polyether resins having the amphoionic group of theformula: ##STR2## As to the details of such resins, reference should bemade to said Japanese Patent Application Kokai No. 129066/83(corresponding U.S. Pat. No. 4,461,870).

The polymerization may preferably and advantageously be carried out byadding a mixture of monomers to an aqueous medium containing theabovesaid amphoionic type resin, in the presence of polymerizationinitiator. The amount of said amphoionic type resin may vary within acomparatively wide range; however, in general, it is in the order of 0.3to 8%, and preferably 0.5 to 6%, by weight of the total amount ofmonomers to be copolymerized. The crosslinking monomer may constitute0.01 to 20%, most preferably 0.1 to 10%, by weight of the totalpolymerization monomers; however, this is not critical in the presentinvention. What is essential is to present the crosslinking monomer inthe reaction system in sufficient quantity to make the microparticlepolymer insoluble in the combination of film-forming polymer and organicliquid diluent. Usually, said insolubility of the microparticles may bechecked by means of the following test. That is, the microparticles (1part by weight) are shaken for 30 minutes with the organic liquiddiluent (e.g. tetrahydrofuran) (100 parts by weight), the suspension isthen centrifuged at 17,000 r.p.m. for 30 minutes. The supernatant liquidis decanted off and the residual polymer is dried and weighed. Theweight of said polymer is compared with that of the microparticlesoriginally taken. Where the result of this test indicates that themicroparticles are acceptably insoluble in the diluent alone, it can beassumed that the particles will be at least equally insoluble in thecombination of the film-forming polymer and the diluent.

The reaction medium, i.e. water, may be used in an amount so as to givea resinous emulsion of 2 to 65%, preferably 20 to 60%, non-volatilesolid content. In order to assist the solubilization of said amphoionicresin, a quantity of basic material equivalent to the acid value maypresent in the reaction medium. As the basic materials, use can be madeof alkali metal hydroxides, ammonia and organic amines, but for reasonof volatile property and giving no residual inorganic ions in the formedcoating, preference is given to ammonia or organic amine. The aqueousmedium may also contain a water miscible organic solvent, if desired. Asthe polymerization initiator, any of the members known to be useful inthe related technical field may satisfactorily be used, includingorganic peroxides such as benzoyl peroxide, t-butyl peroxide and cumenehydroperoxide, organic azocompounds as azobiscyanovaleric acid,azobisisobutyronitrile, azobis-(2,4-dimethyl)-valeronitrile, andazobis-(2-amidinopropane) hydrochloride, inorganic water soluble radicalinitiators as potassium persulfate, ammonium persulfate, sodiumpersulfate and hydrogen peroxide, and redox type initiators comprisingthe combination of said inorganic water soluble radical initiator andsodium pyrosulfite, sodium hydrogen sulfite or bivalent Fe ion. They maybe used each alone or in combination. Such initiator may be previouslycontained in the reaction medium or may be added to the reaction systemsimultaneously with the addition of constituent monomers. The amount ofinitiator is usually in a range of from 0.05 to 5%, preferably 0.1 to3%, by weight of the total monomers to be copolymerized. If desired, anormal chain transfer agent as lauryl mercaptan, hexyl mercaptan and thelike may be present in an appropriate amount.

By the adoption of said emulsion polymerization techinique, a stablydispersed, milky or creamy resinous emulsion can be obtained, theaverage diameter of contained microparticles being in the range of from0.02 to 0.5μ. When removing water from the emulsion by spray drying,solvent replacement, azeotropic distillation, centrifugal separation,filtering, drying and other appropriate means, a somewhat coheredpolymeric mass having a maximum diameter of about 40μ can be obtained,which, however, is never a fused mass. Such product may be used directlyor after being pulverized as the microparticles in the present highsolid coating composition. For the purpose of drying said emulsion, aspray drying is most preferred because of the ease of operation and theparticle diameter obtainable therewith.

Afternatively, the polymeric microparticles used in the invention may beprepared in a non-aqueous organic solvent by the so-called NAD methodusing the resin having an amphoionic radical as dispersion stabilizingagent. In that method, there is used a low polarity organic solvent thatcan be dissolved the monomers but not the polymer, like aliphatichydrocarbons as exemplified by hexane, heptane and octane.

As previously mentioned, the amphoionic type resins used in theinvention may be of the organic solvent soluble type and for the reasonsof specific surface activity and electrochemical properties possessed,they are useful as effective dispersing and stabilizing agents in suchcircumstances. The reaction initiator to be used, operational detailsand after treatment in regard to NAD method are well known in the artand hence no particualr mention would be required thereon. It will besufficient to say that even in NAD method, the polymeric particleshaving an average diameter of 1 to 40μ can be obtained. In eithermethod, α,β-ethylenically unsaturated monomers and crosslinking monomersare successfully copolymerized in the presence of said amphoionic typeresin, and without the necessity of using any additional emulsifier, andmicroparticles of polymer crosslinked to the extent that they areinsoluble in the organic liquid diluent are obtained. Furthermore, thusformed microparticles are always accompanied, through physical adhesion,by the amphoionic type resin used, which has an excellent affinity tovolatile organic liquid diluent to be used in the coating compositionand therefore, when added to the composition for film-forming polymer,crosslinking agent and organic liquid diluent, they can be maintained ina stabilized state of dispersion in that system.

The inventors have previously found that an improved coating compositionbeing excellent in application characteristics and storage stability andcapable of resulting in a coating with excellent appearance can beobtained by using composite resin particles each of which comprises agranular body portion of crosslinked polymer having a mean diameter of0.01 to 10μ and a number of linear polymer chains, a part of therespective chain penetrating into the inside of said granular bodyportion and the remaining part extending outwardly therefrom, in thecombination with a common film-forming resin, and applied for a patentunder Japanese Patent Application No. 267019/84 (now publicly opened asKokai No. 223015/86).

In this invention, the crosslinked polymer microparticles (C) may havethe similar composite structure, too. At that time, a mixture ofα,β-ethylenically unsaturated compound bearing an anionic or cationicfunctional group and other α,β-ethylenically unsaturated compound(s) ispolymerized in the presence of crosslinked polymer particles with a meandiameter of 0.01 to 10μ, by using a solution polymerization means toobtain composite resin particles each of which comprises a granular bodyportion of crosslinked polymer having a mean diameter of 0.01 to 10μ anda number of linear polymer chains having anionic or cationic functionalgroups, a part of the respective chain penetrating into the inside ofsaid granular body portion and the remaining part extending outwardlytherefrom. Thus obtained composite resin particles are used as thecrosslinked polymer microparticles (C).

In the present invention, thus obtained microparticles are added to thefilm-forming polymer (A) organic liquid diluent (B) crosslinking agent(D) system to give a high solid coating composition. The compoundingratio of these four components may be freely varied in comparativelywide ranges depending on the application and the desired effect;however, in general, 50 to 99.5 parts by weight of the film-formingpolymer (in terms of solid content) are compounded with 50 to 0.5 partsby weight of the microparticles. The amount of crosslinking agent is, ingeneral, 5 to 100 parts by weight per 100 parts of the aggregated weightof the film-forming polymer and the microparticles (i.e. polymericcomponents) and the amount of organic liquid diluent is about 10 to 2000parts by weight per 100 parts of the aggregated weight of said polymericcomponents and the crosslining agent (i.e. solid matters). If desired,the coating composition of this invention may further contain otherconventional additives such as antioxidants, UV-absorbers, surfacemodifiers, viscosity modifiers, pigments, metal flakes and the like. Nospecific techiniques or apparatus are required for the preparation ofthe coating composition of this invention.

In the present coating composition, the film-forming polymer (A) and thecrosslinked polymer microparticles (C) bear the functional groups whoseelectric charges are opposite to each other and the film-forming polymer(A) is strongly adsorbed on the surface of the respective crosslinkedpolymer particles (C) through the electrostatic attraction force actingtherebetween.

Therefore, thus obtained microparticles can hardly agglomerate into massin a coating composition comprising the film-forming polymer (A), whichis believed to be the main reason why the present coating composition isexcellent in storage stability and can provide a coating with excellentgloss and appearance.

The present coating compositions do possess an adequate fluidity forspray coating, and are able to make a thicker coating without the fearof sagging. The coatings exhibit high gloss and excellentfilm-performance.

Since the compositions contain neither a low molecular weightemulsifying agent nor a graft polymer dispersing agent usually found inthe conventional compositions, which agents may cause deterioration offilm properties and because the catings are only formed from theresinous material integrally crosslinked and hardened, from theabovesaid microparticles, film-forming polymer and crosslinking agent,they are quite useful as decorative coatings for automobile and otherarticles. This is due to the characteristics of the resulting filmsi.e., excellent durability and good appearance.

The invention shall be now more fully explained in the followingExamples, which, however, should not be taken as being limitative in anysense. In these Examples, unless otherwise being stated, parts and % areby weight.

REFERENCE EXAMPLE 1

(a) Prepartion of polyester resin having an amphoionic group:

Into a 2 liters flask fitted with stirrer, nitrogen inlet pipe,thermoregulator, condenser and decanter, were placed 134 parts ofbishydroxyethyl taurine, 130 parts of neopentylglycol, 236 parts ofazelaic acid, 186 parts of phthalic anhydride and 27 parts of xylene,and the mixture was heated, while removing the formed waterazeotropically with xylene. The temperature was raised to 190° C. inabout 2 hours from the commencement of reflux and stirring anddehydration were continued until the acid value reached 145. Then thereaction mixture was allowed to cool to 140° C. and while maintainingthe same temperature, 314 parts of Cardura E-10 (glycidyl versatate,manufactured by Shell Chem. Co.) were added dropwisely in 30 minutes.After continuing stirring for 2 hours, the reaction was completed. Thethus obtained polyester resin had an acid value of 59, hydroxyl numberof 90 and number average molecular weight of 1054.

(b) Preparation of anionic polymer microparticles (1):

Into a 1 liter reaction vessel fitted with stirrer, condenser andthermoregulator, were placed 281 parts of deionized water, 30 parts ofthe polyester resin obtained in the preceding paragraph (a) and 3 partsof dimethyl ethanolamine and the mixture was heated, while stirring, to80° C. to get a clear solution. To this, were added 1.0 part ofazobiscyanovaleric acid dissolved in a combined solution of 45 parts ofdeionized water and 0.9 part of dimethyl ethanolamine, and thendropwisely a mixture of 20 parts of methyl methacrylate, 30 parts ofn-butyl acrylate, 70 parts of styrene, 15 parts of 2-hydroxyethylacrylate, 5 parts of methacrylic acid and 60 parts of ethyleneglycoldimethacrylate was added over 60 minutes. After completion of saidaddition, 0.5 part of azobiscyanovaleric acid dissolved in a combinedsolution of 15 parts of deionized water and 0.4 part of dimethylethanolamine were added and the mixture was stirred at 80° C. foradditional 2 hours to obtain an emulsion having a solid content 40%, andparticle diameter 0.12μ. By subjecting the emulsion to a spray drying,polymer microparticles (1) were obtained.

(c) Preparation of anionic polymer microparticles (2):

Into a similar reaction vessel as used in the preceding paragraph (b),were placed 218 parts of deionized water, 1 part of ethyleneglycoldimethacrylate, 1.5 parts of methyl methacrylate, 4.5 parts of styrene,2.0 parts of isobutyl methacrylate, 1.0 part of 2-hydroxyethyl acrylateand 2.0 parts of 2-acrylamide-2-methylpropane sulfonic acid and themixture was, while stirring, heated to 70° C. To this, was added asolution of 1 part of ammonium persulfate in 10 parts of deionized waterand the mixture was reacted for 10 minutes. Thereafter, a mixture of 9parts of ethyleneglycol dimethacrylate, 25.5 parts of methylmethacrylate, 35.5 parts of styrene, and 18 parts of n-butyl acrylatewas dropwise added in 2 hours. Finally, a solution of 0.2 part ofammonium persulfate in 5 parts of deionized water was added and thereaction was continued for 4 hours to obtain an emulsion having a solidcontent of 40% and a mean diameter of the microparticles contained of0.35μ. The emulsion was subjected to a spraying drying to obtained thepolymer microparticles (2).

(d) Preparation of cationic polymer microparticles (3):

Into a similar reaction vessel as used in the preceding paragraph (b),were placed 281 parts of deionized water 30 parts of the poyester resinobtained in (a) and 3 parts of dimethyl ethanolamine and the mixture washeated under stirring at 80° C. to obtain a clear solution. Next, asolution of 2,2'-azobis (2-amidinopropane) dihydrochloride in 45 partsof deionized water was added, and then a monomer mixture of 20 parts ofmethyl methacrylate, 30 parts of n-butyl acrylate, 70 parts of styrene,15 parts of 2-hydroxyethyl acrylate, 5 parts of dimethylaminoethylmethacrylate and 60 parts of ethyleneglycol dimethacrylate was dropwiseadded in 60 minutes. Thereafter, a solution of 0.5 part of 2,2'-azobis(2-amidinopropane) dihydrochloride in 15 parts of deionized water wasadded and the combined mixture was stirred at 80° C. for 2 hours toobtain an emulsion having a solid content of 40% and a mean diameter ofcrosslinked polymer microparticles of 0.09μ. The emulsion was subjectedto a spray drying to obtain the polymer microparticles (3).

(e) Preparation of anionic, film-forming polymer:

Into a 3 liters flask fitted with stirrer, thermoregulator, condenserand nitrogen gas inlet tube, were placed 200 parts of butylcellosolveand it was heated under stirring to 100° C. Next, a mixture of 300 partsof styrene, 245 parts of methyl methacrylate, 400 parts of n-butylacrylate, 25 parts of 2-hydroxyethyl acrylate and 30 parts of acrylicacid, a mixture of 20 parts of azobisisobutyronitrile, 80 parts ofmethyl ethyl ketone and 40 parts of xylene, and 10 parts of laurylmercaptane were simultaneously and dropwise added. Thereafter, asolution of 5 parts of azobisisobutyronitrile in a mixture of 20 partsof methyl ethyl ketone and 10 parts of xylene was dropwise added and thecombined mixture was reacted at 100° C. for 2 hours. The temperature wasthen lowered to 70° C. and the mixture was added with 650 parts ofxylene to obtain an acrylic resin varnish. Number average molecularweight of the acryl resin contained was 8320.

(f) Preparation of cationic, film-forming polymer:

The similar procedures as stated in the preceding paragraph (e) wererepeated, excepting substituting 30 parts of dimethyl aminoethylmethacrylate for the monomer mixture of said (e), to obtain in acrylicresin varnish. Number average molecular weight of the resin was 9510.

EXAMPLE 1

20 parts of the anionic polymer microparticles obtained in the precedingReference Example 1(b) were dispersed in a mixture of 42 parts ofxylene, 30 parts of methyl isobutyl ketone and 8 parts of n-butanol.While stirring the dispersion in a disper, 280 parts of the cationicacryl resin varnish obtained in the preceding Reference Example 1(f) and120 parts of U-van 20 SE-50 (trademark, butylated melamine of MitsuiTohatsu Kagaku) were added to obtain a clear coating composition. Afteradjusting viscosity of said composition to a value which was appropriateto spraying (#4 Fordcup viscosity 25 seconds), thus obtained dilutedcomposition was applied onto a tinplate vertically held, by a spray gunso as to give a dry thickness of 40μ. After standing for 5 minutes, thecoated plate was baked at 140° C. for 25 minutes. PDG value (imagesharpness value prescribed by Nihon Shikisai Kenkyu-sho and measured byPortable Distinctness of image Glossmeter manufactured by TokyoKodensha) of the coating was 0.8 and thus the coating appearance wasexcellent.

EXAMPLE 2

The same procedures as stated in Example 1 were repeated exceptingsubstituting 20 parts of the anionic polymer microparticles obtained inthe preceding Reference Example 1(c) for the anionic polymermicroparticles of Example 1. PDG value of thus obtained coating was 0.7and thus the coating appearance was excellent.

EXAMPLE 3

20 parts of the cationic polymer microparticles obtained in thepreceding Reference Example 1(d) were dispersed in a mixture of 42 partsof xylene, 30 parts of methyl isobutyl ketone and 8 parts of n-butanol.While stirring said dispersion in a disper, 280 parts of the anionicacryl resin varnish obtained in the preceding Reference Example 1(f) and120 parts of U-van 20 SE-60 were added and mixed well to obtain a clearcoating composition. After adjusting the viscosity, thus obtaineddiluted composition was applied onto a tinplate and baked, as inExample 1. PDG value of the coating was 0.8 and thus the coatingappearance was excellent.

COMPARATIVE EXAMPLE 1

The same procedures as stated in Example 1 were repeated exceptingsubstituting 20 parts of the cationic polymer microparticles obtained inthe preceding Reference Example 1 (c) for the anionic polymermicroparticles of Example 1. PDG value of the coating was 0.5. Thus, thecoating appearance was fairly good but inferior to that of the coatingof Example 1.

COMPARATIVE EXAMPLE 2

The same procedures as stated in Example 3 were repeated exceptingsubstituting 20 parts of the anionic polymer microparticles obtained inthe preceding Reference Example 1 (b) for the cationic polymermicroparticles of Example 3. PDG value of the coating was 0.5. Thus, thecoating appearance was judged to be good by visual inspection butinferior to that of the coating of Example 3.

EXAMPLE 4 AND COMPARATIVE EXAMPLE 3

    ______________________________________                                                           Ex. 4                                                                              Comp. Ex. 3                                           ______________________________________                                        resin varnish of Ref. Example 1 (e)                                                                125    125                                               microparticles of Ref. Example 1 (d)                                                                5     --                                                microparticles of Ref. Example 1 (b)                                                               --      5                                                n-butanol modified melamine varnish                                                                25     25                                                xylene               15     15                                                n-butanol             5      5                                                carbon black pigment  6      6                                                ______________________________________                                    

Into a paint conditioner, the abovesaid materials were placed and thecontent was mixed well by using steel beads for 2 hours to obtain acoating composition, in Example 4 and then in Comp. Example 3. The bakedcoatings were prepared as in Example 1. PDG value and gloss value of therespective coating are shown in the following Table.

    ______________________________________                                                      PDG   gloss (60° gloss)                                  ______________________________________                                        Example 4       0.8     98                                                    Comp. Example 3 0.4     92                                                    ______________________________________                                    

What is claimed is:
 1. A coating composition comprising(A) afilm-forming polymer having at least one functional group capable ofreacting with a crosslinking agent and having at least one cationic oranionic functional group, (B) a volatile organic liquid diluent in whichthe polymer (A) is dissolved, (C) crosslinked polymer microparticles,bearing at least one anionic or cationic functional group whose electriccharge is opposite to that of the at least one cationic or anionic groupof said film-forming polymer (A), which are insoluble in the combinationof the film-forming polymer (A) and the diluent (B) and are maintainedin a stabilized state of dispersion therein, and (D) a crosslinkingagent dissolved in the diluent (B).
 2. The composition according toclaim 1, wherein the weight ratio of the film-forming polymer (A) to thecrosslinked polymer microparticles (C) is 50:50 to 99.5:0.5, the weightratio of (A)+(C) to the crosslinking agent (D) is 100:5 to 100:100 andthe weight ratio of (A)+(C)+(D) to the organic liquid diluent (B) is100:10 to 100:2000.
 3. The composition according to claim 1, wherein thefilm-forming polymer is selected from the group consisting of acrylicresin and alkyd resin, having both hydroxyl group and carboxyl or aminogroup.
 4. The composition according to claim 1, wherein the volatileorganic liquid diluent is selected from the group consisting of analiphatic hydrocarbon, an aromatic hydrocarbon, an ester, a ketone, analcohol and any combination thereof.
 5. The composition according toclaim 1, wherein the crosslinking agent is an aminoplast resin or apolyisocyanate compound.
 6. The composition according to claim 1,wherein the polymer microparticles (C) are crosslinked copolymermicroparticles having an anionic functional group selected from carboxylgroup, sulfonic acid group or phosphoric acid group, or a cationicfunctional group selected from amino group or ammonium group.
 7. Thecomposition according to claim 1, wherein the polymer microparticles (C)are crosslinked copolymer microparticles obtained by thecopolymerization of at least 3 α,β-ethylenically unsaturated monomers,the first member being α,β-ethylenically unsaturated compound with ananionic or a cationic functional group, the second member being apolyfunctional monomer having two or more ethylenic unsaturations in itsmolecule or the combination of two ethylenically unsaturated compoundseach having mutually reactive functional groups, and the third memberbeing an α,β-ethylenically unsaturated monomer other than the abovesaidfirst and second members, in an aqueous or organic medium in thepresence of a dispersing agent, having a mean diameter of 0.01 to 40μ.8. The composition according to claim 7, wherein the dispersing agent isa resin having in its molecule an amphoionic group of the formula:##STR3## in which R is a member selected from substituted orunsubstituted alkylene having 1 to 6 carbon atoms or phenylene and Y is--COOH or --SO₃ H.
 9. The composition according to claim 1 furthercontaining a metallic or non-metal pigment.